1. Field of the Invention
The present invention relates generally to a printing apparatus. More specifically, the invention relates to a printing apparatus which has a construction to efficiently use a power source capacity.
2. Description of the Related Art
Associating with spreading of information processing apparatuses, such as copy machines, wordprocessors, computers and the like, and communication apparatus, printing apparatus which performs digital image printing in an ink-jet system, as one of image forming (printing) apparatus for outputting information, such as a character, an image and so forth, processed by the foregoing apparatuses, are spreading. In such ink-jet printing apparatus, it is typical to employ a printing head, in which a plurality of ink ejection openings and liquid passages are integrated, as a plurality of printing elements in order to improve a printing speed or so on.
Among these ink-jet printing apparatus, a serial printer which is one of low cost and can be down-sized easily, holds high demand for a personal use. In the printer, it is possible that an amount of information varies significantly depending upon content of an image data fed from a host computer, and an uneven distribution of printing dot density is present depending upon printing portions in a predetermined region, such as one page of a printing paper, to cause significant fluctuation of necessary electric power for driving a printing head and printing amount. When there are provided with a power source having sufficiently large capacity and a circuit withstanding for inputting and outputting of such large capacity power source to enable even printing despite of the fluctuation of electric power or the like, the printer becomes bulky and expensive.
On the other hand, in general printing, high density printing to be printed by a maximum printing amount of the head is quite a little in a whole image, and most part of the image can be printed by a low density driving output. Therefore, in printers or the like which are intended to realize a lower price and a smaller size, there has been proposed a method in which, instead of employing the power source having large capacity, an amount of electric power to be used within an unit period is restricted by controlling a printing manner according a number of printing dots to be printed on a given size of region, and whereby, in case of the general image data, a printing quality is compensated even in a portion having high printing density.
For example, Japanese Patent Application Publication No. 41114/1987 discloses a printing apparatus which is arranged a plurality of printing elements along a paper feeding direction and performs printing by scanning each printing elements in a direction perpendicular to the paper feeding direction for performing printing. The disclosed printing apparatus comprises printing signal generating means for generating a printing element driving signal for driving the printing elements for expressing a character pattern of each character with dividing characters in one line into a plurality of blocks per n in number of columns, counting means for taking the printing element driving signal from the driving signal generating means as a counting input and for counting number of necessary printing dots for printing each character in each block, detecting means for detecting levels of the counted values per each block counted by the counting means among a preliminarily set plurality of levels, and selecting means receiving an output of the detecting means for selecting a printing speed of one line depending upon the levels of the counted values of the counting means in the block, at which number of printing dots in one line becomes maximum.
While the technology disclosed in the above-identified publication has been proposed in view of a wire-dot system or line printer, it reduces power consumption per unit period by lowering the printing speed for one line.
As another method, there is a method to divide data for a region to be printed once into a plurality of fractions and scan the printing head for a plurality times for the same region. For instance, in Japanese Patent Application Publication No. 47290/1994, there is a disclosure "when a dot duty detecting region, in which dot duty exceeds a predetermined dot duty, is present, the corresponding printing line is printed dividingly for a plurality of times". Then, as a particular method of printing for a plurality of times, an example of printing with dividing for twice printing by the odd sequential number of printing elements and by the even sequential number of printing elements, as shown in FIG. 1A.
As the method to divide the region to be printed in one line by the printing head into two divided regions, methods shown in FIGS. 1B and 1C are also considered. FIG. 1B shows a method, in which for printing elements in a head, all of the printing elements in the head is divided into an upper half block and a lower half block to perform a first scan for printing only by the upper half block and a second scan for printing only by the lower half block. On the other hand, in FIG. 1C, with a mutually complementary two printing masks (here in a form of a grid), printing for an image region D is completed by twice of printing scan. In any cases, the mutual difference between the methods illustrated in FIGS. 1A to 1C is merely in how printing of the image region D is divided into two.
Further, the number of division in these method is not limited to two. Even in the above-identified publication, there is a statement "it is not specified to twice, but can be printed dividingly for more than or equal to twice." It should be noted that there is not a difference in terms of the plurality of times of printing scan for the image region.
However, when the foregoing two conventional methods are applied for an ink-jet printing apparatus, they may encounter the following problems.
When the technology disclosed in the former Japanese Patent Application Publication No. 41114/1987 is applied for a printing apparatus of an ink-jet system, an image is formed with lowering a speed of a carriage mounting the head and a driving frequency of the head, depending upon the number of the printing dots. In this case, in the ink-jet system, a precision of a hitting position and an ejection amount per one dot are affected by the carriage speed and the driving frequency to cause a difference of density between a region printed at low speed and a region printed at a normal speed to make unevenness perceptible on the printed image. While such fluctuation of density may be adjusted by ejection amount control or the like, the apparatus becomes large scale, and a low cost printer cannot be adapted.
Even by the dividing method disclosed in the later Japanese Patent Application Publication No. 47290/1994, which includes a derivative example as shown in FIG. 1, an influence may be perceptible in the image density. Namely, as shown in FIGS. 1A and 1C, when an imaging region is divided into a uniform pattern for a plurality of times of printing scans, it has been observed by the inventors that the density of the image formed by a plurality of times of printing scans becomes higher than the printing image formed by once of printing scan. Even in this case, the density of the printed image only in the region dividedly printed becomes high, as in the former prior art. In contrast to this, it has been confirmed that, by the method as shown in FIG. 1B, in which upper and lower blocks of the printing elements of the head are dividingly driven, the foregoing problem will not be caused.
FIG. 2 is an illustration showing a printing condition, in which the method of FIG. 1B is applied. Here, a head, in which sixty-four ejection openings are arranged, is employed to perform printing divided into twice by respective thirty-two ejection openings in a power saving mode.
In FIG. 2, the printing in the first scan shows the case where printing can be performed by both of the upper and lower half blocks (by all ejection openings). After printing scan by all of sixty-four ejection openings, paper feeding is performed for an amount corresponding to sixty-four ejection openings. Next, in the second printing scan, since it is detected that the number of the printing dots is greater than or equal to a predetermined number, it is indicated that printing is performed only by the block 1. It should be noted that paper feeding after printing scan is not performed. Subsequently, in the third printing scan, printing scan is performed with remaining thirty-two ejection openings of the block 2 of the head. Thereafter, paper feeding for sixty-four ejection openings is performed. Then, the fourth and fifth scans are also performed for forming image in the similar manner to those of the second and third scans. By progressing printing, with data of any density distribution, the image can be completed with relatively small power source capacity.
However, in the foregoing prior art, when a certain predetermined number of printing dots is exceeded, a control for switching into a given power saving mode sufficiently restricting power consumption, is performed. Therefore, it cannot be said that the power source can be efficiently utilized within an allowable range of use of the power source. Namely, an operational mode enters into the power saving mode even if number of dots slightly exceeds a threshold value, it becomes difficult to use the power close to a maximum capacity of the power source capacity, in practice to merely prolong printing period, significantly.